I don't have the necessary equipment to test the Krell components in their current-mode (CAST) operation. Therefore, I performed all the technical tests using conventional voltage-mode drive, in both balanced and unbalanced operation.

With its volume control set to "151," the Evolution 202 preamplifier offered maximum voltage gains of 11.8dB in full balanced mode, 5.75dB in unbalanced mode. Both modes preserved absolute polarity; ie, were noninverting. The volume control's unity-gain setting was "101" for balanced operation, "124" for unbalanced. The unbalanced input impedance was a usefully high 38k ohms in the bass and midrange, dropping slightly but inconsequentially to 33k ohms at 20kHz. The balanced input impedance was exactly twice the unbalanced, as expected. The output impedance was low, at 45 ohms balanced, 22.5 ohms unbalanced.

The Evolution 202 offered a very wide bandwidth, its response down by just a quarter of a dB at 200kHz (fig.1). This didn't vary with volume-control setting or load impedance, and was identical in the balanced and unbalanced modes. There is a small rise in output at infrasonic frequencies, presumably a function of the DC servo. DC offset was negligible. Channel separation (fig.2) was superb, at better than 110dB below 5kHz (balanced) and 198kHz (unbalanced), with the volume control set to its maximum. Note, however, the slight peaks at 180Hz and 300Hz in this graph, possibly due to magnetic coupling from the power supply, which had to be placed directly under the preamp chassis for the testing due to the short umbilical cable. Even so, the levels of these spuriae are way too low to be audible, though they did reduce the worst-case, unweighted, wideband signal/noise ratio to a still good 82.2dB (input shorted, volume control maximum, ref. 1V output). A-weighting increased this figure to a superb 95.7dB.

The Krell preamp could swing very high output voltages, even into low impedances. Fig.3 reveals that the balanced output didn't clip (defined as 1% THD) into 100k ohms until 16V RMS, with the unbalanced output clipping at 8V. These figures are significantly higher than that required to drive the Evolution 600 power amplifier into clipping. More important, you can see in this graph how the actual distortion remains below the noise floor until 4V (balanced) and 2V (unbalanced), which suggests that the preamp's gain architecture has been sensibly optimized. I plotted the balanced THD percentage against frequency at 4V, the level suggested by fig.3, but there was no difference in the preamp's behavior across the audioband, and the THD remained very low even into the punishing 600 ohm load (fig.4).

And even with its very low level of distortion, the spectral content of that distortion was benign, with only the second and third harmonics rising above the residual level of my signal generator (fig.5). Intermodulation distortion was also low (fig.6).

I preconditioned the Evolution 600 power amplifier by running it at 200W into 8 ohms for an hour. At the end of that period, the THD+noise percentage had dropped from 0.0159% to 0.0064%, but the internally housed heatsinks were too hot to touch. The chassis itself was around 55°C at the rear, but cooler than that toward the front. Like the Evolution 202, the Evolution 600 monoblock was measured only in voltage mode. Its voltage gain (into 8 ohms) was a little lower than average, at 25.25dB, meaning that it needs almost 4V RMS from both its balanced and unbalanced inputs to be driven to its maximum output power. It preserved absolute polarity, and the input impedance was a high 90k ohms unbalanced, 200k ohms balanced.

My measurement of the 600's output impedance was 0.124 ohm, which is higher than the specified figure. My measurement does include 6' of cable, though this should account for no more than a small fraction of the difference. But this is still low enough that the modification of the amplifier's frequency response by the interaction between this impedance and that of the loudspeaker is negligible, at ±0.1dB (fig.7). This graph also reveals that the power amplifier has a wide small-signal bandwidth, though not as wide as the Evolution 202 preamplifier. The –3dB point lay at 116kHz, with a very-well-defined 10kHz squarewave the result (fig.8). The bandwidth did decrease slightly into 2 ohms, but was the same for balanced and unbalanced drive.

Fig.9 reveals that the Evolution 600 was a powerhouse, clipping with continuous drive at 610W into 8 ohms (27.85dBW), 1190W into 4 ohms (27.75dBW), and 2.2kW into 2 ohms (27.4dBW). I don't hold my AC wall voltage constant for these tests; at 125.8V with the amplifier quiescent, it had dropped to 121.8V with the amplifier clipping into 4 ohms, 119V with it clipping into 2 ohms. With its S/N ratio (input shorted, ref. 1W into 8 ohms) of 81.7dB (wideband, unweighted) improving to 92.3dB when A-weighted, the Evolution 600 offers a superbly wide dynamic range that well exceeds that of the 16-bit CD medium.

I plotted the 600's THD+N percentage against frequency at 11V output, the level at which, as indicated in fig.9, the distortion harmonics begin to rise out of the (low) noise floor. Even so, as fig.10 reveals, though the percentage does rise with decreasing load impedance, it remains very low and constant with frequency at audio frequencies. And, as with the 202 preamplifier, the harmonic content of the 600's distortion remains low in order, almost entirely second- and third-harmonic in nature (fig.11), with no AC-supply spuriae visible even at high powers (fig.12). Intermodulation distortion was also very low, even at almost 800W into 4 ohms (fig.13)!

Having had on my test bench in recent months a number of, shall I say, idiosyncratically engineered products, it was a pleasure to measure such a well-engineered pair of components as the Krell Evolution 202 and 600.—John Atkinson